DbMigration supports altering UniqueConstraints and foreign keys (#1279)

* NEW: Ebean reads version from pop.properties and prints it at start up

* DDL generation and migration can append a header and platformMigration is prepared to append epilog & prolog to scripts

* FIX: Set identity only, if it is not the platform default type

* Updated reference scripts

* ADD basic support for foreign key migration

* DbMigration detects alter foreign keys

* tests

src/main/java/io/ebeaninternal/dbmigration/migration/ObjectFactory.java

@@ -194,5 +194,17 @@ public class ObjectFactory {
     return new DdlScript();
   }
 
+  /**
+   * Create an instance of {@link CompoundUniqueConstraint }
+   */
+  public AddUniqueConstraint createCompoundUniqueConstraint() {
+    return new AddUniqueConstraint();
+  }
 
+  /**
+   * Create an instance of {@link AddUniqueConstraint }
+   */
+  public AlterForeignKey createAlterForeignKey() {
+    return new AlterForeignKey();
+  }
 }